A vibration damper of said type is known from DE 44 41 047 C1. On a piston guided axially within a cylinder barrel, there are provided pressure-dependent damping valves for an exchange of fluid between two working chambers in the rebound phase and in the compression phase. A pilot controller acts such that a controllable pressure is built up in pilot-control chambers. The pilot-control pressure forces the valve disks of the damping valves into their closed position. By means of a bypass duct system, fluid is conducted out of the working chambers into the pilot-control chambers, and thus the pressure there is increased. The pilot-control pressure is regulated by means of an externally adjustable pilot-control valve arranged in the bypass duct system. An elastic element forms the partition between the pilot-control chamber and the working chamber.
A further vibration damper, which operates in accordance with the same principle, is known from DE 101 04 640 C1. In this, the pilot-control chambers are formed by pressure chambers. Said pressure chambers are formed by in each case one pressure chamber housing and one seal, which lies in movable fashion against a damping valve. A pilot-control valve controls the pressure in the pilot-control chambers. An outlet of the pilot-control valve opens into the working chambers.
The valve disks generally impart a degressive damping characteristic to the vibration damper at high piston speeds in the case of which a lift-off of the valve disks from the valve seat occurs. Here, the opening cross section becomes larger with increasing speed, which gives rise to the degressive characteristic.
In parallel with the damping by means of the valve disks, one or more bypass ducts in the piston form a throttle between the working chambers, which throttle imparts a progressive component to the damping characteristic of the vibration damper. The influence of said progressive component is of relevance in particular at low piston speeds in the case of which the valve disks still bear fully against the valve seat. Such a bypass duct may be formed by means of a notch in the valve disk or in the valve seat, such that a flow through the fluid leadthroughs from one working chamber into the other working chamber is possible despite the valve disks bearing fully against the valve seat. However, a bypass duct of said type basically acts in both throughflow directions, such that, as a result, the progressive component can be set only to the same extent for both throughflow directions.
The dimensioning of the bypass cross sections through the notches on the valve disk or the valve seat necessitate adapted valve disks or pistons, which makes it necessary to stock a high number of different disks and pistons.
WO 2010/122102 A1 discloses a vibration damper which is intended to be distinguished by particularly smooth opening of the valve disks. For the compression and rebound side, multiple valve seat regions are formed in each case for one valve disk, which valve seat regions can be supplied with hydraulic fluid to varying degrees. In the steady state, the same hydraulic pressure prevails at all valve seat regions, such that the valve disk opens quickly in the event of further pressurization. In the dynamic state, the pressure cannot be maintained in some valve seat regions owing to a relatively small inflow cross section of the associated hydraulic duct. Said valve seat regions thus duly assist a fast opening of the valve disk; it is then the case that, when the disk is open, said valve seat regions have no influence or only little influence on the further opening of the valve. The vibration damper does not provide any bypass ducts in the piston.
It is an object of the present invention to further develop the generic vibration damper such that the damping characteristics at low piston speeds can be set separately for the two throughflow directions in an extremely simple manner.